Using your throwhand pull either
to the right or left and give just a slight tug as you do so. The yo-yo
as it returns grabs the string using its spin and actually tightens
and loosens the string the same as a UFO would.
(Tighter if the hand moves towards your throwhand side and looser if
it moves towards your non-throwhand side.) Catch the yo-yo as it returns
to your hand.